This invention relates to high strength stainless steel alloys and, in particular, to a precipitation-hardenable, martensitic stainless steel alloy having a unique combination of strength, ductility, toughness, and machinability.
Aerospace material specification AMS 5659 describes a 15Cr-5Ni precipitation hardenable, corrosion resistant steel alloy for use in critical aerospace components. AMS 5659 specifies minimum strength and ductility requirements which the alloy must meet after various age-hardening heat treatments. For example, in the H900 condition (heated at about 900 F. (482 C.) for 1 hour and then air cooled), a conforming alloy must provide a tensile strength of at least 190 ksi (1310 MPa) in both the longitudinal and transverse directions together with an elongation of at least 10% in the longitudinal direction and at least 6% in the transverse direction. However, products manufactured to meet that specification typically lack the ease of machinability desired by component fabricators.
As the alloy specified in AMS 5659 continues to be used in many structural components for aerospace applications, a need has arisen for an alloy that meets all of the mechanical requirements of AMS 5659, but which also provides superior machinability. It is generally known to add certain elements such as sulfur, selenium, tellurium, etc. to stainless steel alloys in order to improve their machinability. However, the inclusion of such xe2x80x9cfree-machining additivesxe2x80x9d, without more, will adversely affect the mechanical properties of the alloy, such as toughness and ductility, to the point where the alloy becomes unsuitable for the critical structural components for which it was designed. Consequently, a need exists for a precipitation-hardenable martensitic stainless steel having good ductility, toughness, and notch tensile strength to be useful for critical applications and which also provides superior machinability compared with alloy compositions currently utilized for fracture-critical components.
The present invention is directed to a precipitation-hardenable martensitic stainless steel which provides mechanical properties (tensile and notch strength, ductility, and toughness) that meet the requirements of AMS 5659 and which also provides significantly better machinability compared to the known grades of 15Cr-5Ni precipitation-hardenable stainless steels. The broad, intermediate, and preferred weight percent compositions of the alloy according to this invention are set forth in the following table.
The foregoing tabulation is provided as a convenient summary and is not intended thereby to restrict the lower and upper values of the ranges of the individual elements for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the ranges can be used with one or more of the other ranges for the remaining elements. In addition, a minimum or maximum for an element of a broad, intermediate, or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate composition. Here and throughout this specification the term xe2x80x9cpercentxe2x80x9d or the symbol xe2x80x9c%xe2x80x9d means percent by weight unless otherwise specified.
The interstitial elements carbon and nitrogen are restricted to low levels in this alloy in order to benefit the machinability of the alloy. Therefore, the alloy contains not more than about 0.030% each of carbon and nitrogen and preferably not more than about 0.025% of each of those elements. Carbon and nitrogen are strong austenite stabilizing elements and limiting them to levels that are too low leads to the formation of undesirable amounts of ferrite in this alloy. Therefore, at least about 0.010% each of carbon and nitrogen is preferably present in the alloy.
This alloy contains a controlled amount of sulfur to benefit the machinability of the alloy without adversely affecting the ductility, toughness, and notch tensile strength of the alloy. To that end, the alloy contains at least about 0.005% and preferably at least about 0.007% sulfur. Too much sulfur adversely affects the ductility, toughness, and notch tensile strength of this alloy. Therefore, sulfur is restricted to not more than about 0.015% and preferably to not more than about 0.013% in this alloy.
At least about 14.00% and preferably at least about 14.25% chromium is present in the alloy to provide an adequate level of corrosion resistance. However, when chromium is present in excess of about 15.50% the formation of undesirable ferrite results. Therefore, chromium is restricted to not more than about 15.50% and preferably to not more than about 15.25% in this alloy.
At least about 3.50%, preferably at least about 4.00%, nickel is present in the alloy to maintain good toughness and ductility. Nickel also benefits the austenite phase stability of this alloy at the low levels of carbon and nitrogen used in the alloy. The strength capability of the alloy in the aged condition is adversely affected when more than about 5.50% nickel is present because of incomplete austenite-to-martensite transformation (i.e., retained austenite) at room temperature. Therefore, this alloy contains not more than about 5.50% nickel.
At least about 2.50%, preferably at least about 3.00%, copper is present in this alloy as the primary precipitation hardening agent. During the age hardening heat treatment, the alloy achieves substantial strengthening through the precipitation of fine, copper-rich particles from the martensitic matrix. Copper is present in this alloy in amounts ranging from 2.50 to 4.50% to provide the desired precipitation hardening response. Too much copper adversely affects the austenite phase stability of this alloy and can lead to formation of excessive austenite in the alloy after the age hardening heat treatment. Therefore, copper is restricted to not more than about 4.50% and preferably to not more than about 4.00% in this alloy.
A small amount of molybdenum is effective to benefit the corrosion resistance and toughness of this alloy. The minimum effective amount can be readily determined by those skilled in the art. Too much molybdenum increases the potential for ferrite formation in this alloy and can adversely affect the alloy""s phase stability by promoting retained austenite. Therefore, while this alloy may contain up to about 1.00% molybdenum, it preferably contains not more than about 0.50% molybdenum.
A small amount of niobium is present in this alloy primarily as a stabilizing agent against the formation of chromium carbonitrides which are deleterious to corrosion resistance. To that end the alloy contains niobium in an amount equivalent to at least about five times the amount of carbon in the alloy (5xc3x97%C). Too much niobium, particularly at the low carbon and nitrogen levels present in this alloy, causes excessive formation of niobium carbides, niobium nitrides, and/or niobium carbonitrides and adversely affects the good machinability provided by this alloy. Too many niobium carbonitrides also adversely affect the alloy""s toughness. Furthermore, excessive niobium results in the formation of an undesirable amount of ferrite in this alloy. Therefore, niobium is restricted to not more than about 0.30%, better yet to not more than about 0.25%, and preferably to not more than about 0.20%. Those skilled in the art will recognize that tantalum may be substituted for some of the niobium on a weight percent basis. However, tantalum is preferably restricted to not more than about 0.05% in this alloy.
A small but effective amount of boron may be present in amounts up to about 0.010%, preferably up to about 0.005%, to benefit the hot workability of this alloy.
The balance of the alloy composition is iron except for the usual impurities found in commercial grades of precipitation hardening stainless steels intended for similar use or service. For example, aluminum is restricted to not more than about 0.05% and preferably to not more than about 0.025% in this alloy because aluminum can form aluminum nitrides and aluminum oxides which are detrimental to the good machinability provided by the alloy. Other elements such as manganese, silicon, and phosphorus are also maintained at low levels because they adversely affect the good toughness provided by this alloy. The composition of this alloy is balanced so that the microstructure of the steel undergoes substantially complete transformation from austenite to martensite during cooling from the annealing temperature to room temperature. As described above, the constituent elements are balanced within their respective weight percent ranges such that the alloy contains not more than about 2 volume percent (vol. %) ferrite, preferably not more than about 1 vol % ferrite, in the annealed condition.
The alloy according to this invention is preferably melted by vacuum induction melting (VIM), but can also be arc-melted in air (ARC). The alloy is refined by vacuum arc remelting (VAR) or electroslag remelting (ESR). The alloy may be produced in various product forms including billet, bar, rod, and wire. The alloy may also be used to fabricate a variety of machined, corrosion resistant parts that require high strength and good toughness. Among such end products are valve parts, fittings, fasteners, shafts, gears, combustion engine parts, components for chemical processing equipment and paper mill equipment, and components for aircraft and nuclear reactors.
The unique combination of properties provided by the alloy according to the present invention will be appreciated better in the light of the following examples.